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Decay of uranium-238 and thorium-232 will continue to provide enough heat for plate tectonics for at least the next few billion years. Their half-lives are 4.5 and 14.05 billion years. But how much is too little heat to power plate tectonics is difficult to say. Also happening before much of the remaining U-238 and Th-232 decays is loss of water by evaporation into space. Lack of water for lowering the melting point of rock will make subduction more difficult, and may stop the plates.

... simple decay of radioactive isotopes provides about half of the Earthís heat, and will continue to provide heat for billions of years. The rest of the heat is left over from the planetís formation.

As you haven't posted in over 10 years it's fair to assume that you may have forgotten the rules we have here on Against The Mainstream posts. For one, they may only be posted in the ATM sub-forum, where special rules apply (rule #13). Second, part of rule 13 is that you may present an ATM theory only once, and need moderator permission to post on that topic again. Since you already had a thread on an "expanding earth" hypothesis, you may not post such again without express moderator permission.

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Subduction zones have key characteristics that help geologist and seismologist identify them. The first is mountain formation. Subduction zones always have mountain ranges caused by plate subduction. The next is volcanic activity as a plate is subducted the pressure and heat turns it into magma. These pockets of magma find paths to the surface and create volcanoes. A good example is the subduction zone near Chile. The final sign is deep marine trenches. These are the best evidence of a subduction zone as they are visible evidence of the crease formed by subduction of a plate. The most famous is the Mariana Trench.

And then there is common sense - we see that the ocean ridges are being continuously formed by volcanic activity, we see material flowing away from them (e.g. magnetic stripes in the sea floor), so where does this material end up?

If Silver and Behn are right (and they have been criticized), tectonics is in any case an intermittent process, and has already stopped for a while a billion years ago when the supercontinent Rodinia was formed.